This invention relates to electronic ballast circuits for high intensity discharge (HID) lamps and more particularly to an electronic ballast circuit for effecting a relatively rapid activation of a HID lamp.
Generally, high intensity discharge (HID) lamps, such as mercury-arc lamps, for example, have a negative resistance impedance where its maintaining voltage is a function of the arc tube's temperature. Ordinarily, a ballast inductor is employed which limits the current flow with respect to lamp voltage. As a result the power available at the lamp is limited and a relatively long warm-up period is required before the desired lighting is attained. Moreover, the ballast inductor type circuitry is relatively inefficient and subject to poor power regulation in the event of line voltage fluctuations.
In an attempt to overcome the above-mentioned disadvantages, electronic ballast circuitry evolved and a lengthy discussion of such circuitry is contained on pages 594-606 of an article entitled "Power Transistors Application Note AN-3616" issued by RCA Solid State Division. Therein, electronic ballast circuits such as a ringing-choke converter, a push-pull inverter, and a switching regulator are discussed in detail.
In the above-mentioned discussion, it is noted that the ringing-choke converter has a relatively poor operating efficiency while the push-pull inverter suffers from relatively poor regulation and requires an excessive amount of magnetic components. Thus, the preferred electronic ballast circuit is the switching regulator type of circuitry.
As to the switching regulator type of ballast circuitry, it has been a common practice to provide circuitry to gradually decrease the current flow through the lamp as the voltage across the lamp increases. As a result, power in an amount sufficient to effect lighting eventually appears at the lamp. However, it has been found that such circuitry does leave something to be desired as to complexity of the circuit as well as an undesired relatively long warm-up period before the desired lighting is achieved.